The present invention is directed to the creation of grating structures in photosensitive optical waveguides such as optical fibres.
Optical fibre communication has seen a tremendous growth in the past several years due to increased traffic flow. One particularly important device that is employed in communication systems, particularly in WDM systems, is the in-fibre Bragg grating. The grating is formed as a periodic modulation in the refractive index along a portion of the length of optical fibre. The periodic modulation is normally created by exposing the fibre to a UV beam or the like, and one method of forming the grating utilises the interference pattern from two interfering beams of light so as to create the grating.
International Patent Application No. PCT/AU96/00782, entitled xe2x80x9cRing Interferometer Configuration for Writing Gratingsxe2x80x9d discloses one form of vibration insensitive grating writing system which utilises a Sagnac type loop to maintain vibrational stability. Although the implementation of such system has been found to give a relatively high grating performance, it is always desirable to provide for improved grating systems that allow more complex and accurate grating designs to be written and so allow for improved grating performance.
Broadly defined, the present invention provides a method of forming a grating structure having a predetermined length in a photosensitive waveguide. The method comprises the steps of:
(a) moving the waveguide relative to a light source in the longitudinal direction of the grating,
(b) directing coherent light from the light source perpendicularly to the direction of relative movement of the waveguide and irradiating spaced-apart regions of the waveguide to form the grating structure.
(c) modulating the light and, as a separate step, passing the light through a shutter in its passage between the light source and the waveguide, and
(d) controlling the shutter to effect shuttering of the light of synchronisation with the relative movement of the waveguide during formation of the grating structure.
The light from the light source may be directed through a phase mask following its passage through the shutter and, so, be controlled to create an interference pattern for irradiating the spaced-apart regions of the waveguide. In forming the interference pattern, two first order beams that are produced by the phase mask may be projected in mutually opposite directions around a Sagnac loop.
In an alternative arrangement, a single beam of light may be focused to irradiate successive regions of the waveguide as the waveguide is moved relative to the light source and the shutter is controlled synchronously to operate and pass light to the fibre. In this case the beam will be focused to a region having a width substantially less than 500 nanometers, this being less than the period of a typical grating.
The fibre may be moved with a substantially constant velocity or at a velocity that is varied so as to produce a chirp in the grating structure.
Also, the duty cycle of the shutter may be controlled during its operation to effect apodisation of the grating structure.